Inorganic coloring pigments are already widely employed in many industries, especially in those of paints, plastics and ceramics. In such applications, the properties consisting, inter alia, of thermal and/or chemical stability, dispersibility (ability of the product to disperse correctly in a given medium), intrinsic color, coloring power and hiding power constitute some particularly important criteria to be taken into consideration in the choice of a suitable pigment.
Unfortunately the problem is that most of the inorganic pigments which are suitable for applications such as the above and which are actually employed nowadays on an industrial scale generally rely on metals (especially cadmium, lead, chromium and cobalt) the use of which is becoming increasingly strictly regulated, or even prohibited, by the legislations of many countries, bearing in mind, in fact, their toxicity, which is reputed to be very high. It is thus possible to mention more particularly, by way of nonlimiting examples, the case of red pigments based on cadmium selenide and/or cadmium sulphoselenide and for which substitutes based on rare-earth sulphides are now already proposed, and that of green pigments which, for their part, exhibit the disadvantage of containing, in most cases, chromium especially in the form of cobalt chromite, of Victoria Green (chromium-based garnets) or of chromium(III) oxide. The case of cobalt-based blue pigments may also be mentioned.
It can be seen, therefore, that the search for, the development and finally the provision of new substitute inorganic pigments nowadays constitute one of the most important economic and industrial objectives.
Barium and copper silicates, especially of blue or violet color, are furthermore known, but the preparation of these products presents difficulties. In fact, these silicates are generally obtained by grog firing. In this case, because of the high temperatures required by this type of process, melt phases are necessarily involved as intermediates and, as a result, products are obtained essentially in the form of glasses. These glasses exhibit a very dark violet color and their use as colorants is very limited. At the most, in fact, they can be employed for coloring ceramics or fruits because it is difficult to grind them to a particle size of less than about ten microns, a particle size which is necessary for coloring other materials, because such grinding generally entails an excessive change in color, which makes them unusable.
There is therefore an undoubted need for colored pigments based on alkaline-earth metal and copper silicates, of fine particle size, permitting the coloring of a wide range of materials and also exhibiting a wide range of colors, especially in the blue and the violet.